Hello everyone! My daughter is in Cheerleading and was wondering what is the best lens for stopping this fast motion of tumbling, toe touches, and back flips. Our son is in baseball also and would like to do the same thing - stop motion. Cheerleading venues are usually dark with stage lighting on the girls... baseball is played outdoors..

Cheerleading will be extremely difficult - especially with the XTi (poor focus and only iso 1600). You're going to have to use prime lenses. If you can be within 15 feet you can start with the relatively cheap 50mm 1.8 ($85) lens. Much better is the 85mm 1.8 ($370) - good to about 25 feet. The 100mm 2.0 ($370) is good for about 30 feet. After that you jump up to the $1000 135mm 2.0. After that you just don't want to know.

Baseball - the proper lens depends on:

1. Level of play (small field vs. full field rec vs ful field high school, etc)

2. Where you are allowed to shoot from

Normally it would depend on day/night but the truth is with only ISO 1600 you're not going to get any night shots under the lights so I would concentrate on the day shots.

I wonder if flash is possible... (instead of coming out empty handed in the dark) ?

A 580EX is good for 10m at f/5.6, ISO100 (good for about 30ft)
-> This distance can be extended further as the camera's ISO is increased and the lighting might not look 'natural', but that's definitely a possibility if you don't have access to a fast camera/lens (beside one might not want a shallow DOF if a camera has poor focusing capabilty...)

What level of play and where will you be shooting from (behind a fence, in the dugout, etc....) That will drive what focal length lens is needed.

For example - if it's HS baseball and you have to shoot from behind the fence, you'll want something in the neighborhood of a 400-500mm lens. For the same field if you're shooting from on the field you can get infield and shallow outfield shots with 300mm although 400 is better. Pony leagues if you can get close, 300mm might work ok. Teeball and little league, 200mm will get you some shots although 300mm is still preferable.